Question

In which parts of this excerpt from "Dover Beach" by Matthew Arnold does the speaker describe the sorrow and confusion that he wants to escape?
[The Sea of Faith
Was once, too, at the full,] and round earth's shore
Lay like the folds of a bright girdle furl'd.
[But now I only hear
Its melancholy, long, withdrawing roar,]
Retreating, to the breath
Of the night-wind, down the vast edges drear
And naked shingles of the world.

[Ah, love, let us be true
To one another!] for [the world, which seems
To lie before us like a land of dreams,]
So various, so beautiful, so new,
[Hath really neither joy, nor love, nor light,]
Nor certitude, nor peace, nor help for pain;
[And we are here as on a darkling plain]
Swept with confused alarms of struggle and flight,
Where ignorant armies clash by night.

Answer 1

"Dover Beach" is actually a lyric that was written by Matthew Arnold. And based on the excerpts taken from this lyric, the parts that show the author's sorrow and confusion are these: 

"[Hath really neither joy, nor love, nor light,]
Nor certitude, nor peace, nor help for pain;
[And we are here as on a darkling plain]
Swept with confused alarms of struggle and flight,
Where ignorant armies clash by night."

Hope this answer helps.

Answer 2

[But now I only hear

Its melancholy, long, withdrawing roar,]

Retreating, to the breath

Of the night-wind, down the vast edges drear

And naked shingles of the world.